Only Google knows for sure, but it has to do with the number of incoming links, domain age, voodoo and the phase of the moon..
http://www.iprcom.com/papers/pagerank/ This should be good for a read, if you had like to spend some time for calculations
This ought to bein the google section. Anyway, he idea is to get linked by higher raning pages to increase PR.
Google’s patented method for measuring page importance on a scale from 0 - 10, where 10 is the highest. The PageRank algorithm analyzes the quality and quantity of links that point to a page.
PageRank is Google's opinion on how important a page is, represented on a scale of 0 to 10. A page with a PageRank (PR) of 0 means that, in Google's eyes, the page is not important. Conversly, a page with a PR of 10 means that, in Google's eyes, that site is very important. How is PageRank calculated? Continue reading prmenow.com
From that article: So there is still a bit of guesswork involved and nobody knows exactly how PR is calculated....Except for Google..
Nobody is going to tell you except giving you some ideas like PR uses link analysis,count backlinks, surrounding text, content analysis, life of domain etc...